S.-W.Lee et al.Materials Science and Engineering A 464 (2007)76-84 83 System A (a) System B tensile direction System A System B (b) System B System A B 寸 tensile direction B 0 System A System B Fig.12.The schematic model of cooperative grain boundary sliding in alloys with (a)low B volume fraction and (b)high B volume fraction,respectively. 5.Conclusions specimens are primarily of B phase displaying an excellent plasticity.The cavities preferentially nucleate at the o-B (1)This work has demonstrated that Mg-15Al-1Zn alloy could interface. be refined to get a fine and uniform two-phase structure (6)This Mg-15Al-1Zn alloy has low forming stress,low form- directly from the as-cast billets by the reciprocating extru- ing temperature,high forming rate and high superplasticity sion method. and superior specific strength.It is promising for HSRS (2)The Mg-15Al-1Zn alloy possesses superior yield and forming of high performance parts for transportation and ultimate strengths,306 and 376 MPa,respectively,and a 3C products. moderate elongation of 5%.This is due to its fine-grained microstructure and high volume fraction of hard B phase. Acknowledgment (3)The maximum elongation of Mg-15Al-1Zn alloy was over 1610%in company with a high m value of 0.7 obtained at The authors would like to thank the National Science Council 325C and 1x 10-2s-1.The apparent activation energy of the Republic of China,Taiwan,for financially supporting this for superplastic flow is 83.8 kJ/mol and smaller than the research under Contract No.NSC 93-2216-E-007-035. boundary diffusion in Mg.This decrease is attributable to the smaller activation energy of B phase. (4)The excellent HSRSP is related with cooperative grain References boundary sliding (CGBS)of refined two-phase structure enhanced by the active grain boundary sliding of B phase. [1]W.E.Quist,R.E.Lewis,in:M.E.Fine,E.A.Starke (Eds.),Rapidly Solid- (5)B phase tends to flow into the horizontal region by the oper- ified Powder Aluminum Alloys,ASTM Publication,Philadelphia,1984. Pp.7-38. ation of CGBS.Ligaments in the surface of superplastic [2]I.C.Hsiao,J.C.Huang.Metall.Mater.Trans.A33A(2002)1373-1384.S.-W. Lee et al. / Materials Science and Engineering A 464 (2007) 76–84 83 Fig. 12. The schematic model of cooperative grain boundary sliding in alloys with (a) low volume fraction and (b) high volume fraction, respectively. 5. Conclusions (1) This work has demonstrated that Mg–15Al–1Zn alloy could be refined to get a fine and uniform two-phase structure directly from the as-cast billets by the reciprocating extru￾sion method. (2) The Mg–15Al–1Zn alloy possesses superior yield and ultimate strengths, 306 and 376 MPa, respectively, and a moderate elongation of 5%. This is due to its fine-grained microstructure and high volume fraction of hard phase. (3) The maximum elongation of Mg–15Al–1Zn alloy was over 1610% in company with a high m value of 0.7 obtained at 325 ◦C and 1 × 10−2 s−1. The apparent activation energy for superplastic flow is 83.8 kJ/mol and smaller than the boundary diffusion in Mg. This decrease is attributable to the smaller activation energy of phase. (4) The excellent HSRSP is related with cooperative grain boundary sliding (CGBS) of refined two-phase structure enhanced by the active grain boundary sliding of phase. (5) phase tends to flow into the horizontal region by the oper￾ation of CGBS. Ligaments in the surface of superplastic specimens are primarily of phase displaying an excellent plasticity. The cavities preferentially nucleate at the – interface. (6) This Mg–15Al–1Zn alloy has low forming stress, low form￾ing temperature, high forming rate and high superplasticity and superior specific strength. It is promising for HSRS forming of high performance parts for transportation and 3C products. Acknowledgment The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 93-2216-E-007-035. References [1] W.E. Quist, R.E. Lewis, in: M.E. Fine, E.A. Starke (Eds.), Rapidly Solid￾ified Powder Aluminum Alloys, ASTM Publication, Philadelphia, 1984, pp. 7–38. [2] I.C. Hsiao, J.C. Huang, Metall. Mater. Trans. A 33A (2002) 1373–1384.